(1) Field of the Invention
The present invention relates to a microscopic photometric system for use in film thickness measurement, line width measurement or the like. More particularly, the invention relates to a method executed with use of such a microscopic photometric system to measure light reflected by minute test samples, and to a reflected light measuring apparatus.
(2) Description of the Related Art
At a testing stage of a semiconductor manufacturing process, for example, a thickness of silicon oxide film formed on a silicon substrate is measured by first measuring light reflected by a surface of the substrate.
A conventional apparatus used for measuring this reflected light will be described hereinafter in relation to a relative reflectance measuring apparatus.
In this apparatus, light from a light source such as a halogen lamp, deuterium lamp or the like is reflected toward a sample by a half mirror mounted in the body tube of a microscope, and projected through an objective to a surface of the sample. Light reflected by the sample surface and having a predetermined wavelength range travels through the objective and the half mirror to a spectrophotometric unit. This spectrophotometric unit has an electromagnetic shutter disposed at an entrance thereof to open and close the entrance, and a pinhole disposed downstream of the shutter. The light having passed through the pinhole is dispersed by a concave diffraction grating and reaches a one-dimensional solid-state image pickup device. This image pickup device has a signal output connected to a CPU included in a data processor. The CPU also controls opening and closing of the electromagnetic shutter mentioned above.
Methods of measuring a relative reflectance with this apparatus will be described next.
In a first method, a sample to be used as a reference (a silicon substrate without silicon oxide film in this example) is placed on the stage of the microscope first of all. Intensity C(.lambda.) of light reflected by the sample and incident upon the spectrophotometric unit is measured.
Next, the electromagnetic shutter at the entrance is closed to darken the spectrophotometric unit. Then, a dark current D(.lambda.) of the one-dimensional solidstate image pickup device is measured.
A sample to be tested for relative reflectance (a silicon substrate having silicon oxide film in this example) is placed on the stage of the microscope next. Intensity M(.lambda.) of light reflected by this test sample is measured after opening the electromagnetic shutter of the spectrophotometric unit.
Based on the above measurements, relative reflectance R(.lambda.) is derived from the following equation: EQU R(.lambda.)={M(.lambda.)-D(.lambda.)}/{C(.lambda.)-D(.lambda.)}(1)
In a second method, a sample to be used as a reference (a silicon substrate without silicon oxide film in this example) is placed on the stage of the microscope first of all. Intensity C(.lambda.) of light reflected by the sample and incident upon the spectrophotometric unit is measured.
Next, a perfect diffuser plate or the like having a minimal reflectance which may be regarded as 0% is placed on the stage, and light intensity D(.lambda.) is measured.
Then, as in the first method, light intensity M(.lambda.) from a test sample is measured. In this method also, relative reflectance R(.lambda.) is derived from equation (1) above.
The conventional methods described above have the following drawbacks.
In the first method, the dark current of the one-dimensional solid-state image pickup device may be eliminated from light intensities M and C in the above equation (1). However, it is impossible to eliminate stray light (i.e. undesirable light due to causes other than normal refraction or reflection) occurring between the electromagnetic shutter of the spectrophotometric unit and the interior of the microscope body tube and between the objective and the sample. The stray light lowers the precision of relative reflectance measurement.
This problem will particularly be described with reference to FIG. 1.
FIG. 1 is a graph showing relative reflectance R(.lambda.) obtained from the first method above, and relative reflectance T(.lambda.) obtained theoretically. The theoretical relative reflectance T(.lambda.) is derived from a film thickness on the test sample, refractive index of the film, absorption coefficient of the film, refractive index of the substrate, absorption coefficient of the substrate, and so on.
As seen from this graph, the relative reflectance R(.lambda.) influenced by the stray light and the theoretical relative reflectance T(.lambda.) are significantly different in minimal regions.
Such stray light is generated chiefly as a result of the light from the light source scattering when passing through the half mirror in the body tube of the microscope, or reflected by the incident surface of the objective to enter the spectrophotometric unit directly instead of traveling by way of the sample. Particularly where the objective comprises a reflecting objective formed of a plurality of concave or convex reflecting mirrors, light incident on the optical axis of the reflecting objective is subjected to regular reflection to enter the spectrophotometric unit directly. This results in increased influences of the stray light.
The drawback may be mitigated to some extent by applying tufty paper (flockpaper) on inner walls of the body tube of the microscope to scatter light, or by forming recesses (light traps) in axial portions of spherical reflecting mirrors of the reflecting objective to scatter light. However, these provisions cannot remedy the drawback completely.
The second method can remove some of the influences of stray light since the intensity D of light reflected by the perfect diffuser plate corrects reflected light intensities M and C. However, the stray light cannot be removed completely since no perfect diffuser plate has 0% reflectance over a wide range of wavelengths. A complex corrective computation of reflectance would be required to improve the precision of measurement by this method.
Where a light source for emitting ultraviolet light is used, the perfect diffuser plate will deteriorate due to ultraviolet light. It will be extremely difficult to optically maintain 0% reflectance.